1. Field of the Invention
The present invention relates to a display apparatus including a plurality of mutually crossing signal lines and scanning lines, a pixel circuit provided with holding capacitors and switching elements, and light emitting elements, and to a driving method thereof. In particular, the present invention relates to a display apparatus of a current drive type light emitting element including coupling capacitance such as an organic electro-luminescence (EL) element, and a driving method thereof.
2. Description of the Related Art
In the recent years, display apparatuses are being developed energetically. Among such display apparatuses, a display apparatus with an organic EL element for a light emitting element is getting attention. An organic EL element is a self light emitting element that uses the principle in which recombination energy between holes and electrons injected respectively from an anode and a cathode by applying a voltage results in light emission from a light emitting layer made of organic material.
FIG. 24 illustrates a sectional view of a basic organic EL element. An organic EL element 201 has an anode 2502, a light emitting layer 2503 and a cathode 2504, which are stacked on a substrate 2501. Both ends of a drive circuit not illustrated in the drawing are respectively connected with the anode 2502 and the cathode 2504 of the organic EL element 201, so that a voltage generated by the drive circuit is applied to the anode and the cathode. The voltage is applied to the organic EL element 201 and thereby current flows so that light emission takes place at desired brightness.
So far, various methods of driving a display apparatus with an organic EL element have been proposed. However, as resolution of a display and drive speed are enhanced, a demand for increasing the speed of a program of (writing) image data to be input to each pixel is increasing. The driving method is generally categorized into a voltage programming method and a current programming method and, in particular, the slow program speed at the time of low gradation is a problem in the current programming method. On the other hand, it is difficult to compensate variation of current-voltage property of an organic EL element and variation of mobility of a switching element in the voltage programming method.
At first, a display apparatus of the voltage programming method will be briefly described. FIG. 25 is an example of a display apparatus based on the voltage programming method with pixels 107 respectively having a plurality of organic EL elements 201, holding capacitors 301, selection transistors 302 and drive transistors 303 being arranged.
In FIG. 25, a great number of pixels 107 are arranged in a matrix form to configure a display region. Here, in order to simplify the drawing, an example of pixel arrangement for two rows and two columns covering the i-th row to the i+1-th row and the i-th column to the i+1-th column is illustrated. Scanning lines 202 are wired respectively for the pixel circuits 101 in the display region. The scanning signals X(i) to X(i+1) are input to the scanning lines 202 in sequence. Thereby, each pixel 107 is selected for each row. In addition, the signal lines 103, which supplies each pixel with the image data, that is, brightness data Y(i) to Y(i+1), are wired.
In the following description, the pixel circuit 101 will be described by exemplifying the pixel circuit of the pixel (i, i) for the i-th row and i-th column. Here, as for the pixel circuit of the other pixels will be provided with the completely same circuit configuration. In addition, the organic EL element 201 is used as the display element and a thin film transistor is used as the switching element in the pixel circuit.
As illustrated in FIG. 25, the pixel circuit 101 includes a selection transistor 302 for selecting a pixel 107, a holding capacitor 301 for holding a data voltage and a drive transistor 303 for driving an organic EL element 201. The holding capacitor 301 is a capacitor in which an image data for displaying an image is programmed as a voltage and the voltage is held until the next image data is programmed by the signal of a scanning line. The brightness data is given in a voltage form from the signal line 103. A current corresponding with the data voltage flows in the organic EL element 201.
As the specific relation of connection, the anode 2502 of the organic EL element 201 is connected with the power supply voltage 305 (hereinafter to be referred to as Vdd). The drive transistor 303 is brought into connection between the cathode 2504 of the organic EL element 201 and a common ground line 304. The holding capacitor 301 is brought into connection between the gate of the drive transistor 303 and the common ground line 304. The selective transistor 302 is brought into connection between the signal line 103 and the gate of the drive transistor 303. The gate is connected with the scanning line 202.
In the display apparatus by the above described voltage programming method, various circuit configurations and driving methods are proposed for compensating the property variation of the organic EL element, the selective transistor and the drive transistor.
In the case of the voltage programming method, the threshold voltage variation of the organic EL element, the selection transistor and the drive transistor can be compensated by adding transistors and holding capacitor for compensation.
However, it is difficult to compensate the current-voltage property variation of the organic EL element and the mobility variation of the transistor. In order to compensate the variations, a method of driving the display apparatus by the current programming method including a constant current source outside the pixel circuit is proposed.
Since the voltage of the holding capacitor is determined by causing a data current for obtaining desired brightness to flow in one of the transistor and the organic EL element in the pixel circuit, the current programming method will be able to compensate the property variation of the organic EL elements and the transistors more accurately.
FIG. 3 is a drawing illustrating a pixel of a display apparatus of a current programming method proposed by Nathan et al in U.S. Patent Application Publication No. 2007/080908 specification.
This pixel circuit comprises a switching circuit 401 including two selective transistors T1 and T2 and a current mirror circuit 402 including a reference transistor T3 and a drive transistor T4. The control terminals (gates) of the reference transistor T3 and the drive transistor T4 are both connected with a terminal of the holding capacitor. One main electrode (source) of each of the transistors T3 and T4 is connected with the organic EL element 201.
Here, the transistors T1 to T4 are n-type thin film transistors.
A first technological advantage of the pixel circuit herein is the possibility of splitting loads born by the selective transistors T1 and T2 and the drive transistor T4 since this pixel circuit is a current mirror circuit. That is, since the current can flow in the organic EL element 201 only through the drive transistor T4 from the power supply voltage Vdd 305, the electricity consumption can be made small. A second technological advantage is the possibility of compensating the property variation of the organic EL element 201 since current programming can be carried out while a data current 403 is flowing in the organic EL element 201.
A current programming method of the pixel circuit illustrated in FIG. 3 will be described briefly. At first, the selection transistors T1 and T2 are selected with the signal of the scanning line 202 to be activated. In synchronization, a constant current source not illustrated in the drawing supplies the signal line 103 with a predetermined data current 403 so that the electric charge is charged in the holding capacitor 301 through the selective transistor T2. When the holding capacitor 301 is charged to reach a predetermined voltage, the reference transistor T3 and the drive transistor T4 are activated so that a current starts to flow in the organic EL element 201. Here, the organic EL element 201 also has a coupling capacitor. Therefore, the coupling capacitor here is charged so that the predetermined data current 403 does not flow in the organic EL element 201 until a terminal reaches a predetermined voltage.
Subsequently, even if the current programming operation is finished so that the selective transistors T1 and T2 and the reference transistor T3 are inactivated with the signal of the scanning line 202, the drive transistor T4 maintains the ON state with the voltage of the holding capacitor 301. Accordingly, the current corresponding with predetermined data current 403 can continue to flow in the organic EL element 201 through the drive transistor T4 from the power supply voltage Vdd 305 until the next cycle current program.
However, the pixel circuit illustrated in FIG. 3 gives rise to a problem as follows.
A reason here is that, at the time of low gradation, charging to the holding capacitor and the coupling capacitor of the organic EL element cannot catch up with the programming time, resulting in charging insufficiency. In the above described current program, at the time of low gradation, microcurrent has to charge the holding capacitor and the coupling capacitance of the organic EL element. Consequently it takes time for programming. Accordingly, at the time of low gradation, the holding capacitor and the coupling capacitor of the organic EL element gets short of charging. Therefore, accurate gradation display cannot be obtained but black floating appears in the panel display screen, giving rise to a problem.
In order to solve the problem to give rise to shortage in charging at the time of low gradation in a current programming method, a technique called precharge is proposed as prior art.
Precharge is the technique of applying a predetermined voltage to a signal line before starting the current programming operation and, thereby, charging the capacitor component in advance. For example, U.S. Pat. No. 6,310,589 by Nishigaki et al proposes a drive circuit of an organic EL element including a charging circuit for precharge as illustrated in FIGS. 26 to 28.
FIG. 26 illustrates a pixel 107 of a passive matrix organic EL element 201 illustrated in FIG. 27, a constant current source 501 connected to the signal line 103 and a charging circuit 502 for precharge.
Here, the passive matrix drive circuit will be described briefly with FIG. 27. As illustrated in FIG. 27, the passive matrix drive circuit is configured by a plurality of organic EL elements 201, a plurality of signal lines 103 and a plurality of scanning lines 202. In order for a pixel 107 to emit light at predetermined brightness, a voltage can be applied between the mutually crossing signal line 103 and scanning line 202 so that a predetermined current flows in the organic EL element 201.
Getting back to FIG. 26, the description will go on. Each signal line 103 is provided with the constant current source 501 and the charging circuit 502 in FIG. 26 in the panel peripheral circuit outside the pixel circuit. The operation of the drive circuit in FIG. 26 will be described. At first, in synchronization with the output of the constant current source 501, the output of the pulse generator 503 will get High to apply a precharging voltage V 504 from the charging circuit 502 to the signal line 103. The precharge voltage can charge the coupling capacitor of the organic EL element 201 in short time and can shorten the time up to the start of light emission of the organic EL element 201.
FIG. 28 illustrates a timing chart of the drive circuit in FIG. 26. FIG. 26 illustrates a passive matrix drive circuit. Therefore, when the signal of the scanning line 202 is Low, a voltage is applied to the organic EL element 201. During the initial precharging period, the output of the pulse generator 503 gets High to activate the switch SW of the charging circuit 502. Thereby, the drive waveform of the signal line 103 rapidly rises to reach the precharging voltage V 504 to charge the organic EL element 201. Thereafter, during a constant current drive period, the output of the pulse generator 503 gets Low to supply the signal line 103 with the data current only from the constant current source 501.
However, the present inventors have found out that the time for the organic EL element to start light emission is not so shortened as the passive matrix is shortened even if a method of this precharge is applied to the drive circuit of the active matrix in FIG. 3, giving rise to a problem.
The pixel 107 in FIGS. 26 and 27 is replaced with a pixel comprising the switching circuit 401 illustrated in FIG. 3, the current mirror circuit 402 and the organic EL element 201. The signal of the scanning line is changed either from High to Low or from Low to High. Then, precharge can be carried out with a timing chart in FIG. 28.
However, in the case of the active matrix, the precharge voltage is applied not to the coupling capacitor of the organic EL element directly but to the holding capacitor 301. Therefore, it takes time to charge the organic EL element with the terminal-to-terminal voltage. Consequently, the time until starting light emission is not so shortened as the passive matrix is.
Unless the terminal-to-terminal voltage of the organic EL element is charged to a certain extent before the current signal flows in, charging takes time. The variation of the terminal-to-terminal voltage of the organic EL element for that period changes the source-to-drain voltage of the reference transistor T3 to enable no accurate data current to flow. Consequently, the time for the current programming will get longer. Thus, charging the coupling capacitor of the organic EL element is slow in the pixel circuit of the organic EL element of the active matrix and current programming takes time.
The gate voltages of the reference transistor T3 and the drive transistor T4 that are connected to the organic EL element are low at the time of low gradation. At that time, the current supplied to the organic EL element from the reference transistor T3 and the drive transistor T4 is limited. Therefore, charging will take longer at the time of low gradation.